A mixture composed of colored fine particles, which comprises at least a binder resin and a colorant, and more fine colloidal silica or the like, which is added as a flowability-imparting agent independently of the colored fine particles, will hereinafter be called a "toner". Two-component developers composed of a toner and a carrier are used widely owing to good quality in images developed thereby.
On the contrary, the two-component developers however involve the following common defects. Namely,
(1) a toner is triboelectrically charged by mutual friction between the toner and a carrier. However, when a two-component developer is used for a long period of time, the surface of the carrier is contaminated with the toner, so that it is impossible to apply sufficient triboelectric charge to the toner;
(2) the toner and carrier must be controlled at a mixing ratio within a fixed range. However, when the developer is used for a long period of time, the mixing ratio is changed outside the fixed range; and
(3) the surface of a photosensitive member is mechanically damaged by iron powder the surface of which is oxidized or glass beads, which are both generally used as carriers.
Therefore, in recent years, various developing processes making use of a magnetic one-component developer, which is free of any carrier and contains magnetic powder in a toner, have been proposed (for example, U.S. Pat. Nos. 3,909,258 and 4,121,931).
However, these known processes are also accompanied by the following defects. Namely,
(1) since the magnetic one-component developer contains a large amount of magnetic powder which is low in electrical resistance, it is difficult to electrostatically transfer an image developed on an electrostatic latent image to a support material such as plain paper. In particular, sufficient performance in transferring cannot be attained under a high-humidity atmosphere;
(2) it is difficult to produce color developers because the magnetic one-component developer contains a large amount of the magnetic powder of a black color; and
(3) since the magnetic one-component developer contains a large amount of the magnetic powder, its fixing capability is lowered as compared to the two-component developer. As a result, the temperature and pressure of a fixing device must be raised, resulting in increased running cost.
In more recent years, the spotlight of attention has been focused on developing processes making use of a one-component developer free of any magnetic powder and high in electrical resistance. As their developing processes, may be mentioned those based on the touchdown or impression development which is described in, for example, U.S. Pat. No. 2,895,847 or 3,152,012, or Japanese Patent Publication No. 9475/1966, 2877/1970 or 3624/1979. These processes use, as a non-magnetic one-component developer, a toner obtained by taking a carrier out of a conventional two-component developer.
Even in this case, it is however impossible to avoid the occurrence of various problems incidental to the non-magnetic one-component developer as described below.
The first problem is an adhering phenomenon of the developer to a photosensitive member and/or the like. In the conventional two-component developers, a large amount of a carrier such as iron powder or glass beads is mixed in addition to a toner. Therefore, if the toner temporally adheres onto a development roll, development blade and/or photosensitive member, this offers no problem because they are polished by the carrier.
However, since only colloidal silica having a particle size as small as 10-20 m.mu. is independently added as a flowability-imparting agent in the toner as the conventional non-magnetic one-component developer, this developer has little polishing effect. Accordingly, when the developer is used for a long period of time, the toner often adheres onto the development roll, development blade and/or photosensitive member, thereby forming a film of the developer.
The formation of such a developer film makes it impossible for the developer to be charged to come into full contact with the development roll or development blade, resulting in insufficient charge of the developer and hence deteriorated quality of the developed image. In addition, the adhesion of the developer to the photosensitive member has offered a problem that it appears as black scumming.
In more recent years, it has therefore been proposed to independently add a fine inorganic powder having a particle size as great as 0.1-10 .mu.m (or a specific surface area as large as 0.2-30 m.sup.2 /g), thereby enhancing the polishing effect (for example, Japanese Patent Application Laid-Open Nos. 32060/1985, 136752/1985, 183664/1986 and 88554/1989).
However, sufficient flowability cannot be imparted by only the fine powders being used in Japanese Patent Application Laid-Open Nos. 136752/1985 and 183664/1986 and it is hence indispensable to use the conventional flowability-imparting agent such as colloidal silica in combination as described in Japanese Patent Application Laid-Open Nos. 32060/1985 and 88554/1989, resulting in the following unavoidable problem.
The second problem is that such developers cannot be used in a recycling system.
Namely, an image formed by a developer after development of an electrostatic latent image on the surface of a photosensitive member is transferred to a support material such as paper. The whole developer used in the development on the photosensitive member is not completely transferred and 20-40 wt.% of the developer generally remains on the photosensitive member. In a copying machine or printer making use of the conventional non-magnetic one-component developer, the untransferred developer remaining on the photosensitive member has been scraped off by a cleaning blade or the like and collected together to thrown away into a waste developer container.
That reason is that when the developer is used in a recycling system, it passes repeatedly through the imageforming process and hence incurs mechanical external force, whereby the flowability-imparting agent (colloidal silica) to exist on the surfaces of the developer particles is embedded in the developer particles and/or caused to fall off.
As a result, its flowability is excellent in the beginning, but is deteriorated as it is used for a long period of time in the recycling system, so that it is impossible to apply the developer onto a development roll to a constant thickness, resulting in uneven image quality. In addition, its charge level is changed and fog is also produced.
Moreover, when the untransferred developer on the photosensitive member is fed by a screw or the like to use it in the recycling system, the developer is compressed to get clogged, adheres to various parts and/or agglomerates. It has therefore been difficult to use it in long-term continuous operation.
The recycle for reuse of a developer is economical in itself because the 20-40 wt.% portion of the developer, which has been scrapped to date, can be reused and no waste developer container is required, and is also desirable for making apparatus such as copying machines and printers small in size.